We have synthesized and characterized a unique hormone derivative with great potential for interdisciplinary studies of morphological and biochemical aspects of the cellular hormone response. Monomeric ferritin conjugated 1:1 with epidermal growth factor (F:EGF) and purified by affinity chromatography is biologically active and binds to cellular receptors with high specificity. Preliminary experiments have demonstrated the value of F:EGF in localization of plasmalemmal EGF receptors. Following incubation of human epithelioid carcinoma cells A-431 with F:EGF at 4 degrees centigrade, electron microscopy reveals ferritin particles in a dispersed pattern on the cell surface. Warming such labeled cells to 37 degrees centigrade prior to fixation allows progressive aggregation, internalization, and degradation of the F:EGF/receptor complex. We propose to use quantitative image analysis of thin section and high-voltage electron micrographs, complemented by biochemical experimentation, to illuminate further details of cell/hormone interactions. Since F:EGF is a physiologic high-resolution specific marker for surface membrane receptors, we also propose to use it in cytological studies of receptor binding, aggregation, pinocytic uptake, intracellular transport and degradation, and recycling or resynthesis.